Bismuth Single Atoms\nRegulated Graphite Felt Electrode\nBoosting High Power Density Vanadium Flow Batteries

Vanadium flow batteries (VFBs) are considered one of\nthe most promising\ncandidates for large-scale energy storage. However, VFBs suffer from\nrelatively low power density due to severe electrochemical polarization.\nHerein, we report Bi single atoms supported by an N-doped carbon-regulated\ngraphite felt electrode (Bi SAs/NC@GF) with high electrocatalytic\nactivity and stability, owing to the greatly improved active sites\nand optimized Bi–N<sub>4</sub> configuration. Electrochemical\nin situ characterization and theoretical calculations elucidate the\ndesolvation process and specific inner sphere reaction mechanism of\n[V(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>/[V(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup>. As a result, a VFB single cell assembled with\nBi SAs/NC@GF achieves a much higher energy efficiency of 81.1% at\n240 mA cm<sup>–2</sup> than NC@GF (70.5%). Moreover, a 5 kW\nVFB stack equipped with Bi SAs/NC@GF is assembled for the first time\nand ran stably for over 400 cycles. This work confirms that a single-atom\ncatalyst is efficient for scalable VFBs with high power density and\nlow cost.